But I understand: he did nothing illegal. My question was: how could I have avoided it.
I can empathize with the OP since I’ve been in a similar position, but to answer your question OP, I would bet that if you had responded by email within that 13 hours, the sub would still be available to you. Not your fault, but this is what happens to sellers who have experienced quite a bit of tire kicking or lack of follow through from potential buyers in the past. I presume that email from the seller was indirectly looking for additional assurances in your level of commitment.
Interesting angle. So I would send nearly a $1000 to a stranger WITH ZERO guarantees because I must TRUST the seller [in response to: You pay first, and then trust that the seller will ship your purchase to you]
Unsure how else it would work with these transactions on Audiogon and USAM. Maybe I’m misinterpreting, but I wouldn’t expect a seller to send me anything until I had paid in full. If meeting in-person, and as a seller, I would request at least a deposit. But that’s me and my experience of being stood up from at least three different potential buyers throughout the years.